In an organic electroluminescence display apparatus (hereinafter referred to simply as organic EL display apparatus) which uses an organic electroluminescence element (hereinafter referred to simply as organic EL element) as a light emitting element, the luminance of the organic EL element is controlled with the value of current flowing through the organic EL element. And similarly as in a liquid crystal display apparatus, also in the organic EL display apparatus, a simple matrix type and an active matrix type are known as driving methods. Although the active matrix type has such a drawback that it is complicated in structure in comparison with the simple matrix type, it has such various advantages as an advantage that an image can be displayed with high luminance.
As a circuit for driving an organic electroluminescence light emitting section (hereinafter referred to simply as light emitting section) which forms an organic EL element, a driving circuit (called 5Tr/1C driving circuit) composed of five transistors and one capacitor is commonly known, for example, from Japanese Patent Laid-Open No. 2006-215213. This conventional 5Tr/1C driving circuit includes, as shown in FIG. 1, five transistors of, as shown in FIG. 1, an image signal writing transistor TSig, a driving transistor TDrv, a light emission controlling transistor TEL—C, a first node initializing transistor TND1 and a second node initializing transistor TND2 and further includes one capacitor section C1. Here, the other one of the source/drain regions of the driving transistor TDrv forms a second node ND2 and the gate electrode of the driving transistor TDrv forms a first node ND1.
It is to be noted that the transistors and the capacitor are hereinafter described in detail.
Further, as shown in a timing chart of FIG. 24, within a [period TP (5)1], a pre-process for carrying out a threshold voltage cancellation process is executed. In particular, when the first node initializing transistor TND1 and the second node initializing transistor TND2 are placed into an on state, the potential of the first node ND1 becomes VOfs (for example, 0 volts). Meanwhile, the potential of the second node ND2 becomes VSS (for example, −10 volts). As a result, the potential difference between the gate electrode and the other one (for the convenience of description, hereinafter referred to as source region) of the source/drain electrodes of the driving transistor TDrv becomes higher than Vth and the driving transistor TDrv is placed into an on state.
Then, within a [period TP (5)2], a threshold voltage cancellation process is carried out. In particular, while the on state of the first node initializing transistor TND1 is maintained, the light emission controlling transistor TEL—C is placed into an on state. As a result, the potential of the second node ND2 changes toward a potential difference of the threshold voltage Vth of the driving transistor TDrv from the potential of the first node ND1. In other words, the potential of the second node ND2 which is in a floating state rises. Then, when the potential difference between the gate electrode and the source electrode of the driving transistor TDrv reaches Vth, the driving transistor TDrv is placed into an off state. In this state, the potential of the second node is substantially (VOfs−Vth). Thereafter, within a [period TP (5)3], while the on state of the first node initializing transistor TND1 is maintained, the light emission controlling transistor TEL—C is placed into an off state. Then, within a [period TP (5)4], the first node initializing transistor TND1 is placed into an off state.
Then, within a [period TP (5)5′], a kind of writing process into the driving transistor TDrv is executed. In particular, while the off state of the first node initializing transistor TND1, second node initializing transistor TND2 and light emission controlling transistor TEL—C is maintained, the potential of a data line DTL is set to a voltage corresponding to an image signal [image signal (driving signal, luminance signal) VSig for controlling the luminance of the light emitting section ELP] and then a scanning line SCL is set to the high level to place the image signal writing transistor TSig into an on state. As a result, the potential of the first node ND1 rides to VSig. Charge based on the variation of the potential of the first node ND1 is distributed to the capacitor section C1, the parasitic capacitance CEL of the light emitting section ELP and the parasitic capacitance between the gate electrode and the source electrode of the driving transistor TDrv. Accordingly, if the potential of the first node ND1 varies, then also the potential of the second node ND2 varies. However, as the capacitance value of the parasitic capacitance CEL of the light emitting section ELP has an increasing value, the variation of the potential of the second node ND2 decreases. Generally, the capacitance of the parasitic capacitance CEL of the light emitting section ELP is higher than the capacitance value of the capacitor section C1 and the value of the parasitic capacitance of the driving transistor TDrv. Therefore, if it is assumed that the potential of the second node ND2 little varies, then the potential difference Vgs between the gate electrode and the other one of the source/drain regions of the driving transistor TDrv is given by the expression (A) given below. It is to be noted that an enlarged timing chart within a [period TP (5)5′] and a [period TP (5)6′] is shown in (A) of FIG. 25.Vgs≈VSig−(VOfs−Vth)  (A)
Thereafter, within the [period TP (5)6′], correction (mobility correction process) of the potential of the source region (second node ND2) of the driving transistor TDrv based on the magnitude of the mobility μ of the driving transistor TDrv is carried out. In particular, while the on state of the driving transistor TDrv is maintained, the light emission controlling transistor TEL—C is placed into an on state, and then when predetermined time (tCor) elapses, the image signal writing transistor TSig is placed into an off state to place the first node ND1 (gate electrode of the driving transistor TDrv) into a floating state. As a result, where the value of the mobility μ of the driving transistor TDrv is high, the rise amount ΔV of the potential (potential correction value) in the source region of the driving transistor TDrv is great, but where the value of the mobility μ of the driving transistor TDrv is low, the rise amount ΔV of the potential (potential correction value) in the source region of the driving transistor TDrv is small. Here, the potential difference Vgs between the gate electrode and the source electrode of the driving transistor TDrv is transformed from the expression (A) into the expression (B) given below. It is to be noted that the predetermined time for executing the mobility correction process (total time (tCor) of the [period TP (5)6′]) may be determined in advance as a design value upon designing of the organic EL display apparatus.Vgs≈VSig−(VOfs−Vth)−ΔV  (B)
By the foregoing operation, the threshold voltage cancellation process, writing process and mobility correction process are completed. Within a later [period TP (5)7], the image signal writing transistor TSig is placed into an off state and the first node ND1, that is, the gate electrode of the driving transistor TDrv, is placed into a floating state while the light emission controlling transistor TEL—C maintains the on state and one (for the convenience of description, hereinafter referred to as drain region) of the source/drain regions of the light emission controlling transistor TEL—C is in a state wherein it is connected to a current supplying section (voltage VCC, for example, 20 volts) for controlling the light emission of the light emitting section ELP. Accordingly, as a result of the foregoing, the potential of the second node ND2 rises, and a phenomenon similar to that which occurs with a so-called bootstrap circuit occurs with the gate electrode of the driving transistor TDrv and also the potential of the first node ND1 rises. As a result, the potential difference Vgs between the gate electrode and the source electrode of the driving transistor TDrv maintains the value of the expression (B). Meanwhile, since the current flowing through the light emitting section ELP is drain current Ids which flows from one (for the convenience of description, hereinafter referred to as drain region) of the source/drain regions to the source region of the driving transistor TDrv, it can be represented by the expression (C). It is to be noted that the coefficient k is hereinafter described.
                                                                        I                ds                            =                            ⁢                              k                ·                μ                ·                                                      (                                                                  V                        gs                                            -                                              V                        th                                                              )                                    2                                                                                                        =                            ⁢                              k                ·                μ                ·                                                      (                                                                  V                        Sig                                            -                                              V                        Ofs                                            -                                              Δ                        ⁢                                                                                                  ⁢                        V                                                              )                                    2                                                                                        (        C        )            
Also driving and so forth of the 5Tr/1C driving circuit whose outline is described above are hereinafter described in detail.
Incidentally, in the mobility correction process, the voltage of the source region of the driving transistor TDrv relies upon the image signal (driving signal, luminance signal) VSig as apparent also from the expression (B) and is not fixed. And, since, in order to raise the luminance of the organic EL element, high current flows through the driving transistor TDrv, the rising speed of the rise amount ΔV of the potential in the source region of the driving transistor TDrv is accelerated.
In other words, since the predetermined time for executing the mobility correction process (total time (tCor) of the [period TP (5)6′]) is a fixed design value, where “white display” is to be carried out on the organic EL display apparatus, that is, where the organic EL element displays high luminance, the rise amount ΔV (potential correction value) of the potential in the source region of the driving transistor TDrv exhibits a quick rise as indicated by a solid line ΔV1 in (B) of FIG. 25. On the other hand, where “black display” is to be carried out, that is, where the organic EL element displays low luminance, the rise amount ΔV (potential correction value) of the potential in the source region of the driving transistor TDrv exhibits a slow rise as indicated by a solid line ΔV2 in (B) of FIG. 25. In particular, where the value of ΔV required where “white display” is carried out is represented by ΔVH, the rise amount ΔV reaches ΔVH in time (tH-Cor) shorter than tCor. On the other hand, where the value of ΔV required where “black display” is carried out is represented by ΔVL, ΔVL is not reached if time (tL-Cor) longer than tCor does not elapse. Accordingly, where “white display” is carried out, the rise amount ΔV becomes excessively great, but where “black display” is carried out, the rise amount ΔV becomes excessively small. As a result, such a problem that the display quality of the organic EL display apparatus is deteriorated occurs.
Accordingly, the object of the present invention resides in provision of a driving method for an organic electroluminescence light emitting period of an organic electroluminescence display apparatus which makes it possible to achieve optimization of a mobility correction process of a transistor which composes a driving circuit in response to an image to be displayed.